Digital imaging systems (e.g., cameras) have quickly become a standard feature for portable devices including portable multimedia players, smart phones, and tablet computers. The image quality expectations from these portable cameras has grown as higher quality and higher megapixel cameras have been incorporated into such small devices.
In most instances the most desirable images are produced when the sensors in those devices can capture fine details in both the bright and dark areas of a scene. In other words, the quality of the captured image is often a function of the amount of detail at various light levels that can be captured. For example, a sensor capable of generating an image with fine detail in both the bright and dark areas of the scene is generally considered superior to a sensor that captures fine detail in either bright or dark areas, but not both simultaneously.
Thus, higher dynamic range becomes an important concern for digital imaging performance. For sensors with a linear response, their dynamic range can be defined as the ratio of their output's saturation level to the noise floor at dark. This definition is not suitable for sensors without a linear response. For all image sensors with or without linear response, the dynamic range can be measured by the ratio of the maximum detectable light level to the minimum detectable light level.
Many prior dynamic range extension methods fall into two general categories: improvement of sensor structure, a revision of the capturing procedure, or a combination of the two. One approach used to extend a camera's dynamic range is to employ a multiple exposure (ME) scheme where each exposure captures a different light range of the scene. ME techniques often suffer from motion artifacts when objects in the scene change position between each exposure frame. ME techniques that use multiple frame captures are not ideal for video or photos with motion unless the frame rate of the camera is high enough to avoid motion artifacts. Thus, an important feature for digital cameras is to have a wide dynamic range image or video capture capability by allowing the camera to capture bright and dark areas of a scene, without such ME technique drawbacks.